1. Technical Field of the Invention
The present invention relates generally to spark plugs for use in automotive vehicles or cogeneration systems. More particularly, the invention relates to an improved structure of a spark plug that has a plurality of center electrodes. The improved structure of the spark plug ensures a high capability of the spark plug to ignite the air-fuel mixture and sufficient strength of ground electrodes of the spark plug.
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a tubular metal shell, an insulator, a center electrode, and a ground electrode.
The metal shell has a male threaded portion for fitting the spark plug into a combustion chamber of the engine. The insulator has a center bore formed therein and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell. The center electrode is secured in the center bore of the insulator such that an end thereof protrudes from the end of the insulator. The ground electrode has a side surface, and is joined to the end of the metal shell such that the side surface thereof is opposed to and spaced from the end of the center electrode so as to form a spark gap therebetween.
In recent years, an increase of compression ratios of internal combustion engines has been pursued for the purpose of increasing power output and improving fuel economies. However, at the same time, such an increase in the compression ratio of an internal combustion engine may cause knocking of the engine, which involves vibrations and damage to the engine.
A possible way of avoiding the engine knocking is to accelerate the combustion within a combustion chamber of the engine so as to complete the combustion cycle of the engine before the engine knocking occurs.
It has been known to be effective to provide a plurality of separate sparks to the combustion chamber, in other words, to initiate the combustion at a plurality of spaced points within the single combustion chamber. By providing more than one spark simultaneously, the time required for the flame to propagate across the combustion chamber can be greatly reduced, thereby effectively accelerating the combustion.
In order to provide a plurality of separate sparks to a single combustion chamber, Japanese Unexamined Patent Publication No. S57-193777 discloses, for example, a spark plug that has a plurality of center electrodes.
On the other hand, an increase in the compression ratio of an internal combustion engine, as described above, may also cause an increase in the required spark voltage (i.e., the electric voltage required for sparking) of a spark plug used for the engine.
Such increase in the required spark voltage implies that it becomes difficult for the spark plug to generate sparks in a spark gap of the spark plug. Thus, instead of normal sparks being generated in the spark gap, “side sparks” can be generated.
The side sparks are sparks which creep from a center electrode of the spark plug along the outer surface of the insulator, and fly to the metal shell of the spark plug. More specifically, the side sparks fly over an air pocket formed between the outer surface of the insulator and the inner surface of the metal shell, thus resulting in engine misfires. Consequently, when the side sparks are generated, the performance of the engine will drop.
Moreover, the recent demand for higher power output of an internal combustion engine has required increasing the sizes of intake and exhaust valves for the engine and securing a water jacket for cooling of the engine. As a consequence, a spark plug to be used for the engine which has a plurality of center electrodes, is required to have a limited size not greater than that of a conventional spark plug which has only one center electrode.
This limitation on the size of the spark plug which even has more than one center electrode necessitates a decrease in an air pocket size of the spark plug; the air pocket size is defined herein as the minimum distance between the inner surface of the metal shell and the outer surface of the insulator on a reference plane that includes the end of the metal shell.
However, in addition to the increase in the required spark voltage of the spark plug as described above, the decreased air pocket size of the spark plug may also cause side sparks in the spark plug.
Accordingly, it is required to keep the air pocket size of the spark plug above a certain level, under the limitation on the size of the spark plug, so as to prevent generation of side sparks.
One may consider, for the purpose of securing the air pocket size of the spark plug, enlarging the inner surface of the metal shell so that it is concentric with the outer surface of the same. However, this will result in a decrease in the wall thickness of the metal shell at the end thereof, so that only those ground electrodes that are thin and have a small cross-sectional area can be joined to the end of the metal shell. Consequently, it becomes impossible to secure sufficient strength of the ground electrodes.
Furthermore, since the inner surface of the metal shell of the spark plug is enlarged in comparison with that of a conventional spark plug having the same size and thermal value, the volume of the air pocket formed between the outer surface of the insulator and the inner surface of the metal shell is accordingly increased. Thus, a considerable amount of combustion gas will flow into the air pocket, thereby transferring more heat to the spark plug. As a consequence, the temperature of the end portion of the spark plug that is disposed in the combustion chamber increases accordingly, resulting in pre-ignition of the air-fuel mixture and performance decrement of the engine.